The present invention relates to a piloting circuit for an inductive load, in particular for a DC electric motor.
More specifically, the subject of the invention is a piloting circuit comprising
a circuit branch which is intended to be connected to a DC voltage supply source in such a way as to receive a voltage of predetermined polarity, and in which the drain-source path of a (first) MOSFET transistor intended to receive an on-off piloting signal at its gate, is disposed in series with the load,
a current recirculation diode connected in parallel to the load, and
protection circuit means operable to prevent damage to the said MOSFET transistor and/or the recirculation diode in the event of polarity reversal of the voltage supply source.
In FIG. 1 there is generally indicated a circuit of this type for piloting a DC electric motor. The circuit comprises a circuit branch 3 connected between the positive pole of a DC voltage supply source 4 such as a battery, and ground. The negative pole of this source is likewise connected to ground. In the circuit branch 3, in series with the electric motor 2, there is disposed the drain-source path of a MOSFET transistor 5 which receives an on-off piloting signal on the gate, such as a pulse width modulated signal (PWM), emitted by a control circuit 6. A diode 7 is connected in parallel with the motor 2 to allow the recirculation of the current when the transistor 5 is switched off.
In FIG. 1 the reference numeral 8 indicates the intrinsic or xe2x80x9cparasiticxe2x80x9d diode of the MOSFET transistor 5. This diode does not create problems if the DC supply voltage applied to the circuit 1 has the correct polarity as is shown in FIG. 1.
The intrinsic diode 8 of the MOSFET transistor 5 does create a problem, however, in the event of polarity reversal of the supply source voltage, as is shown in FIG. 2. As will be appreciated from this Figure, in the event of polarity reversal of the voltage supply source a current I flows through the diode 8 and recirculation diode 7. The magnitude of this current can reach an extremely high value, of hundreds of amps, in that this current is resisted solely by the electrical resistance of the connections between the source 4 and the said diodes, which resistance is in general rather low, of the order of tens of m Ohms. Such current is able to damage or even destroy the transistor 5 and the recirculation diode 7.
Various solutions have been proposed in the prior art to remedy the above indicated problem.
A first solution, shown in FIG. 3, consists in the insertion of a further diode 9 in series with the circuit branch 3, disposed in such a way as to oppose the flow of current in the case of polarity reversal of the supply voltage. However, when the polarity of the supply voltage is correct, the same current as flows in the motor 2 also flows in the diode 9, which can be of the order of several tens of amperes there is therefore a high dissipation of power in the diode 9 and this causes a reduction in the overall efficiency of the system and involves the adoption of provisions to allow this diode to be able to dissipate a high power.
In FIG. 4 there is shown a further prior art solution consisting in disposing a further diode 10 in parallel with the circuit branch 3, with its anode connected to ground, and a fuse 11 between this diode and the voltage source. If the polarity of the supply voltage is reversed the diode 10 and fuse 11 are traversed by a high current which in the limit can xe2x80x9cblowxe2x80x9d the fuse. This solution, although able effectively to protect the piloting circuit 1, does not allow the state of the fuse to be determined after a possible accidental reversal of the supply voltage. In any case, when the fuse has been blown it is necessary to provide for replacement of this latter in order to reinstate the functionality of the piloting circuits.
In the other known solution shown in FIG. 5, between the circuit branch 3 of the piloting circuit 1 and the voltage supply source is interposed the contact 12 of a relay the excitation winding 12a of which is connected to a circuit 13 which detects the polarity of the voltage delivered from the source. If the polarity is correct the circuit 13 excites the winding 12a, which causes closure of the contact 12 allowing supply of the piloting circuit of the motor 2. This solution is expensive in that it requires the use of high performance relays. It is moreover affected by all the known problems to which electromechanical switching devices are subject, such as the possibility of sticking of the movable contact of the relay on the associated fixed contact etc.
An object of the present invention is to provide a piloting circuit of the previously defined type, able to prevent damage to the MOSFET transistor and/or the recirculation diode associated with the load in a simple but reliable way in the event of reversal of the supply voltage polarity.
This and other objects are achieved according to the invention with the piloting circuit the salient characteristics of which are defined hereinafter.